The optical disc is a general term of disc-shaped recording media such as a compact disc, a video disc, a phase-change optical disc, a DVD (digital video disc), and the like, which reflect laser beams on recording surfaces thereof, and read recorded signals by the use of reflected beams.
There are read-only and recordable optical discs. The recordable optical disc includes a WORM(Write Once, Read Many) disc and a rewritable disc. CD-ROMs (Compact Disc Read-Only Memories) are typical of the read-only disc. Manufacturers record data such as video, audio, or characters, computer programs, and the like in the CD-ROMs as signals, and distributes them for sale. Users utilize them by reproducing their recorded contents therefrom. Since the WORM disc with a WORD capability and without capabilities of erasing and altering written data, and the rewritable disc with capabilities of erasing and altering written data, can be used to record data therein and can be accessed randomly by users, and are relatively high-speed and large-capacity recording media, they are suitable for use in recording video data or audio data, or suitable for a storage in a computer system.
In the optical disc, as in the case of a magnetic disc recording medium such as a floppy disc or a hard disc, the recording surface on the disc has concentric circular tracks and linear areas in the radial direction thereof, i.e., sectors, which are managed as unit areas, and address information relating to addresses is recorded in specific recording positions indicated by the addresses.
An optical disc recording and reproducing apparatus according to the prior art which records data in the optical disc and reproduces data therefrom, uses an optical head which converges the laser beams to the recording surface thereby reading a signal recorded therein. To read recorded contents including address information, it is required that the optical head be moved to an appropriate position on the recording surface with high precision. For this purpose, feed back control is performed, including tracking control for positioning the head in a desired position of the disc or control of disc rotation for recording/reproduction, as in the case of the magnetic disc medium.
In addition to these control, the apparatus which uses the optical disc requires focusing control for appropriately converging the laser beams to the recording surface, and laser power control for adjusting intensity of the laser beams. Hence, the optical disc reproducing and recording apparatus according to the prior art obtains a tracking error signal or a focus error signal from the reflected beam, and based on these signals, a servo system performs feedback control of the position of the optical head or the laser power. Likewise, the servo system performs the feedback control to rotation of the disc by the servo system.
The optical disc is basically used to record a digital signal therein as a recording medium. The signal recorded in the optical disc is reproduced by irradiating “pits” formed on the optical disc with a laser beam and reading variations in the intensity of the reflected beam of microscopic signals. In this case, because of optical or electrical low-pass frequency characteristics in a reproducing system, even though data has been digitally recorded in the optical disc, reproduced waveform thereof becomes analog signals having intermediate values.
Accordingly, the optical disc recording and reproducing apparatus according to the prior art which performs the control described above, subjects the signal read from the optical disc to binarization, and performs various types of control processes by using the analog processing system and the digital processing system which handles the binarized digital signal and performs highly precise and high-speed processing.
With recent remarkable development of multi-media or computers, optical disc recording media of higher density and larger capacities have been developed. The higher density recording requires more highly precise control, while a demand for higher-speed processing goes on increasing. Accordingly, there is a need for an optical disc recording and reproducing apparatus which can handle a large-capacity optical disc recording medium that is capable of executing more highly precise control and performing higher-speed processing including control than the prior art apparatus.
As the large-capacity optical disc, there has emerged an optical disc which employs a recording method different from that of a conventional optical disc recording medium. While the conventional optical disc medium generally has grooves along tracks, and contains signals recorded in either of convex and concave portions thereof, some optical discs contain signals recorded in both portions. This type of optical disc generally has address areas in which respective address information is recorded for recording in the convex and concave portions, and further, the address areas have been utilized as those having both areas of convex and concave portions, to increase recording capacities. In this optical disc, the address area has no grooves described above, and therefore, the address area and the other recording area must be controlled in different manners.
However, the optical disc recording and reproducing apparatus according to the prior art, is not capable of performing high-speed and highly precise control for the large-capacity recording medium, and has difficulty in appropriately changing control of the optical disc which has the address area and the other area, the structures of which differ from each other.